Pacemakers and ICDs carefully monitor characteristics of the heart, such as the heart rate, to detect arrhythmias, discriminate among different types of arrhythmias, identify appropriate therapy, and determine when to administer the therapy. The device tracks the heart rate by examining electrical signals that result in the contraction and expansion of the chambers of the heart. The contraction of atrial muscle tissue is triggered by the electrical depolarization of the atria, which is manifest as a P-wave in a surface electrocardiogram (ECG) and as a rapid deflection (intrinsic deflection) in an intracardiac electrogram (IEGM). The contraction of ventricular muscle tissue is triggered by the depolarization of the ventricles, which is manifest on the surface ECG by an R-wave (also referred to as the “QRS complex”) and as a large rapid deflection (intrinsic deflection) within the IEGM. Repolarization of the ventricles is manifest as a T-wave in the surface ECG and a corresponding deflection in the IEGM. A similar depolarization of the atrial tissue usually does not result in a detectable signal within either the surface ECG or the IEGM because it is usually small, coincides with, and is obscured by the R-wave. Note that, although the terms P-wave, R-wave and T-wave often refer to features of the surface ECG, herein the terms are used to also refer to the corresponding signals sensed internally. Also, where an electrical signal is generated in one chamber but sensed in another, it is referred to herein, where needed, as a “far-field” signal. Hence, an R-wave sensed in the atria is referred to as a far-field R-wave.
The sequence of electrical events that represent P-waves, followed by R-waves (or QRS complexes), followed by T-waves can be detected within IEGM signals sensed using pacing leads implanted inside the heart. To help prevent misidentification of electrical events and to more accurately detect the heart rate, the stimulation device employs one or more refractory periods and blanking periods. Within a refractory period, the device does not process electrical signals during a predetermined interval of time—either for all device functions (an absolute refractory period) or for selected device functions (a relative refractory period). That is, the signals in the refractor period are discarded. As an example of a refractory period, upon detection of an R-wave on a ventricular sensing channel (or upon delivery of a V-pulse to the ventricles), a Post-Ventricular Atrial Refractory Period (PVARP) is initiated on an atrial sensing channel. A first portion of the PVARP comprises a post ventricular atrial blanking (PVAB) interval wherein the pacemaker can detect signals on the atrial channel but does not use the signals for any purpose. The PVAB interval is provided to prevent the device from erroneously responding to far-field ventricular events (such as far-field R-waves or far-field ventricular evoked responses (VERs)) on the atrial channel. The PVARP concludes with a relative refractory period during which the pacemaker continues to ignore all signals detected on the atrial channel as far as the triggering or inhibiting of pacing functions is concerned, but not for other functions, such as detecting rapid atrial rates or recording diagnostic information.
Accurate detection of heart rates is required, for example, for the purposes of enabling an AMS system wherein the pacemaker switches from a tracking mode such as DDD to a non-tracking mode such as VDI or DDI mode. More specifically, the conventional pacemaker typically compares a current atrial rate with an atrial tachycardia detection threshold (ATDR) and, if it exceeds the threshold, atrial tachycardia is assumed and the pacemaker switches from the tracking mode to the non-tracking mode. Details regarding AMS may be found in the following patents: U.S. Pat. Nos. 5,441,523 and 5,591,214. Note that DDD, VDI, VVI and DDI are standard device codes that identify the mode of operation of the device. DDD indicates a device that senses and paces in both the atria and the ventricles and is capable of both triggering and inhibiting functions based upon events sensed in the atria and the ventricles. VVI indicates that the device is capable of pacing and sensing only in the ventricles but is only capable of inhibiting the functions based upon events sensed in the ventricles. VDI is identical to VVI except that it is also capable of sensing intrinsic atrial activity. DDI is identical to DDD except that the device is only capable of inhibiting functions based upon sensed events, rather than triggering functions. As such, the DDI mode is a non-tracking mode precluding it from triggering ventricular outputs in response to sensed atrial events. Numerous other device modes of operation are possible, each represented by standard abbreviations of this type.
Thus, an AMS system recognizes when the patient is in an atrial arrhythmia such as atrial tachycardia (AT) and/or atrial fibrillation (AF) and switches from the tracking mode to the non-tracking mode to prevent the device from attempting to track the high atrial rates associated with AT/AF. Problems, however, can arise if some rapid atrial events (arising, e.g., due to atrial flutter or AT/AF) occur during the PVAB interval and hence are not detected by the device, or are only intermittently detected. Due to such “hidden” atrial events, the AMS system can potentially switch back and forth between tracking and non-tracking modes, a condition referred to as “mode switch oscillation.” The result is that the patient is alternately paced at a higher ventricular rate (when in the tracking mode) and at a much lower ventricular rate (when in the non-tracking mode), which is unpleasant for the patient and can also increase patient morbidity. Mode switch oscillation can also yield inappropriate diagnostic data that might mislead tiered atrial arrhythmia therapy systems, such as atrial burst-pacing systems, to falsely conclude that therapy is effective, even if ineffective.
FIG. 1 illustrates mode switch oscillation by way of six stored IEGMs collected by a pacemaker implanted within a patient. The IEGM graphs 2 were recorded over a period of about eighty-seconds. The pacemaker was initially in the DDD mode. The starting time for each individual IEGM graph is shown at the beginning of the traces. (Note, however, that within each individual IEGM graph, only the first six seconds of data for that particular interval is shown. Hence, the entire eighty seconds of IEGM data is not completely illustrated.) Each IEGM graphs includes two traces—an atrial trace recorded in a bipolar sensing configuration (A-tip to A-ring) and a ventricular trace in a unipolar sensing configuration (V-tip to case), as well as various event markers identifying P-waves, R-waves, etc. In particular, “trigger” event codes indicate when a mode switch to a non-tracking mode was triggered due to a high detected atrial rate. For clarity in the figure, the mode switching events are further identified by vertical phantom lines 4 followed by “NON-TRACKING” labels. Following each mode switch to the non-tracking mode, individual AMS event codes 6 are applied to individual beats that are “non-tracked.” (Note that it starts marking the first non-tracking ventricular event after the initial trigger to the first “AMS” beat.) Mode switching events back to the tracking mode are identified by way of vertical phantom lines 8 followed by “TRACKING” labels.
Collectively, the IEGM graphs indicate that the pacemaker went through six mode switch operations from the tracking mode to the non-tracking mode during the overall period of about eighty seconds, with the shortest interval between automatic mode switches being only about eight seconds. A close look at the IEGM traces reveals that the patient experienced AT throughout the entire eighty second interval. However, many of the atrial events (i.e. P-waves) occurred during PVAB intervals and, as such, were not detected by the device for the purposes of atrial rate calculation. These hidden events are marked by circles, such as those identified by reference numeral 9. Since the atrial events were hidden within the PVAB interval, the events were not used in the atrial rate calculations, resulting in significant variations in the calculated atrial rate. The variations caused the device to switch back and forth between the tracking mode and the non-tracking mode, i.e. the aforementioned mode switch oscillation occurred. As such, the underlying atrial arrhythmia was frequently hidden during the non-tracking operating mode due to the PVAB interval.
The PVAB interval is important for managing the potential oversensing of ventricular events on the atrial channel. However, as illustrated in FIG. 1, the PVAB interval can result in inappropriate mode switching, including frequent mode switch oscillations. Accordingly, it would be desirable to provide improved techniques for avoiding inappropriate mode switching, particularly due to hidden atrial arrhythmias, and it is to this end that the invention is primarily directed. Such improved techniques help reduce discomfort and risk to the patient and generally improve the quality of life of the patients.